linux-kernel - Re: [PATCH v6] spi: Add SPI driver for Mikrotik RB4xx series boards

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Date:	Fri, 10 Apr 2015 17:31:15 +0200
From:	Bert Vermeulen <bert@...t.com>
To:	Mark Brown <broonie@...nel.org>
CC:	ralf@...ux-mips.org, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	andy.shevchenko@...il.com, jogo@...nwrt.org
Subject: Re: [PATCH v6] spi: Add SPI driver for Mikrotik RB4xx series boards

On 04/09/2015 11:50 PM, Mark Brown wrote:
> On Thu, Apr 09, 2015 at 11:31:12PM +0200, Bert Vermeulen wrote:
>>  The m25p80-compatible boot flash and (some models) MMC use regular SPI,
>>  bitbanged as required by the SoC. However the SPI-connected CPLD has
>>  a "fast write" mode, in which two bits are transferred by SPI clock
>>  cycle. The second bit is transmitted with the SoC's CS2 pin.
> 
>>  Protocol drivers using this fast write facility signal this by setting
>>  the cs_change flag on transfers.
> 
>> The cs_change flag is used here instead of the openwrt version's
>> spi_transfer.fast_write flag. The CPLD driver sets this flag on a
>> per-transfer basis.
> 
> No, this is broken - it's abusing a standard API in a way that's
> completly incompatible with the meaning of that API which is obviously a
> very bad idea, especially since good practice is to offload the
> implementation of that standard API to the core.  It *sounds* like
> you're just trying to implement two wire mode which does have a standard
> API, please use that.

Can you please advise what kind of solution would be acceptable then? I need
to signal from an SPI protocol driver to an SPI master on a per-transfer basis.

Adding a flag to struct spi_transfer for this one driver, as openwrt does,
seems stupid -- I agree. And sure, using spi_transfer.cs_change is a little
dodgy. But I don't see a standard way to do it otherwise, and I don't really
want to keep trying things until you approve of one. So tell me how you
would do it, and I'll implement it that way. I just want to get this code in.

Also, I have no idea what you mean by two-wire mode. This "fast mode" is SPI
+ one extra pin.


-- 
Bert Vermeulen        bert@...t.com          email/xmpp
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